Method of screening and washing paper pulp



1944- F. J. LANG ETAL METHOD OF SCREENING AND WASHING PAPER PULP FiledJuly 31, 1940 2 Sheets-Sheet 1 Q WEAK b00012 Budd/owe INVENTORS Q, mnuxuwe Y arms: 1?. mum

( ATTO'RN'EY Oct. 17, 1944. F. J. LANG ETAL 2,360,779

METHOD OF SCREENING AND WASHING PAPER PULP Filed July 51, 1940 2Sheets-Sheet 2 OIGESTER .tial' in' connection with the Patented Oct. 17,1944 METHOD OF SCREENING AND WASHING PAPER PULP Frank J. Lang,

Niagara Falls, and George R.

Jaeger, North Tonawanda, N. Y asslgnors to International Paper Company,New York, N. Y., a corporation 01' New York Application July 31, 1940,Serial No. 348,646

6 Claims.

Our invention relates to the washing of pulverulent or fibrous materialsor mixtures of pulverulent and fibrous materials. ing may be carried outmerely to dehydrate such materials or to cleanse them for the purpose ofimproving their color or (and most frequently) to recover fromsuspensions of such materials some of the chemicals which may be insolution in the liquid vehicle.

For the purpose of this description our process and apparatus will bedescribed primarily with reference to the washing of kraft and soda woodpulps in which connection the broadest utility of the invention will. beapparent- We do not, however, intend to limit ourselves to the Washingof these particular pulps, nor, indeed, to the washing of any particularmaterial since our invention has considerable value in.connection withother pulps or pulplike masses or pulverulent solid such, for example,as calcium carbonate sludge.

' One object of our invention is to reduce the amount of power consumedper ton of material treated.

A further object of our invention is to minimize the size and capacity;and therefore the cost, of the auxiliary pumping equipmentessenestablishment and utilization of a vacuum.

A further object of our invention is to mini- -mize foaming due to thechemicals entrapped in the masses handled.

A -further object of our invention, and one having particularly todo'with the washing of 'wood pulps, is to transfer the removal of knotsto a step in advance of the washing process and to eliminateit as a stepsubsequent to the washing.

These and other objects will be madeclear from the following descriptiontaken in connection with the annexed drawings in which Fig. 1 is aschematic view or flow diagram of an alkaline pulp washing systemembodying the principles and apparatus of the invention;

Fig. 2 is a schematic view illustrating a modified arrangement of thepulp washers;

Fig. 3 is a view similar to Fig. 2 but illustrating another modificationof the invention; and

Fig. 4 shows an arrangement which is particularly useful in kraft pulpmills using diffusers to wash the pulp.

In either the kraft or the soda is digested primarily by the applicationof caustic soda and the result is the formation of numerous sodium saltssuch as sodium resinate, sodium process wood Such washoleate, sodiumlinoleate, sodium lignate and other essentially soapy compounds. Whenthe cook is completed the pulp is blown from the. digester and may thenbe given any of several difierent preliminary treatments. These varioustreatments are largely a matter of choice, dictated by the experience ofthe particular plant operator or perhaps by the exigencies of theparticular equipment at any given plant. For example, the pulp may begiven a preliminary drainage to remove a very strong black liquor;

the'pulp may then be rediluted either with weak black liquor or amixture of weak and strong black liquors, all of these treatments beingpreliminary to the washing step with which we are here concerned. On theother hand, the pulp from the digester may, without any drainage, be

subjected to such additional dilution as may be required for ease ofhandling, and the dilution may be done with weak black liquor, strongblack liquor or a mixture of' strong and weak black liquors. In anyevent, the pulp as it is prepared for our treatment is in the form of asuspension of fibre in a fairly concentrated black liquor. This liquorcontains valuable chemicals, but is so laden with soapy material as tobe particu larly susceptible to foaming.

In the ordinary washing process, the pulp is diluted with weak blackliquor to a point where it becomes a suillciently fluid mass to becapable of being picked up by a rotating drum surfaced with a wire meshand subject to'internal suction. This suction draws the liquid throughthe pulp andforms a blanket or mat of fairly dehydrated pulp. the matbeing of considerable thickness and lying on the surface of the drum. Asthe drum'rotates the mat is carried over to the 0pposite side where thesuction is discontinued.

The mat is then delivered off the drum and-falls into a vat where it isagain diluted and the process repeated on a second drui'n. This can becarried out over as many stages as may be deemed necessary to accomplishany desired degree of washing.

It is necessary by previous methods not only to dilute, but to agitatethe pulp between each stage of washing, in order to break the pulp upinto a fluid suspension capable of being picked up by each subsequentdrum. A relatively large quantity of foam is inevitably formed and as aresult the vacuum pumping apparatus connected with each drum must be ofsufilcient size and capacity to handle-not only a very considerablevolume of liquor, but also to handle air entrapped in the foam. As afurther result the power conthere is always present/the danger of unduefoaming due to th /soapy chemicals entrapped in the pulp.

Foaming is objectionable primarily because the foam has a tendency torise out of and over- ,flow the vats, carrying with it valuablechemicals which. are accordingly lost. The foaming tendency is greatestas the pulp approaches the first washing stage and for this reason ithas heretofore been considered impossible to remove the knots andoversized chunks from the pulp prior to the washing operation. Theordinary water through the mat, resulting in non-uniform washingthroughout localized and recurrent areas as the drum progresses. An evengreater 2,800,779 sumption of the pulping system is large and.

The knotter screen is of comparatively coarse mesh and all of theacceptable pulp flows through the mesh and into a vat H, the knots andoversized chunks being retained on the screen and ultimately rejected.In the vat II the accepted pulp is agitated by very slowly movingagitators l8 and flows under a baflle l9 into a vat 20 where it ispicked up by a suction action on a drum 2|.

The pulp forms a relatively thick layer on the surface of the drum andwhile still subjected to suction passes under a number of nozzles 22which are supplied by a, pipe line 23 with weak black liquor obtained ashereinafter set forth.

ill eifect of the presence of the knots (or uncooked chips) lies in theporous, absorbent character of the particles themselves. They aresaturated with the black cooking liquor, but due.

' So lorrg as the knots and uncooked chips remain in the mass, theentire mass will be pollutedbli liquor exuded from these particles.

While still subjected to a suction the pulp mat may be passed under anumber of nozzles 24 which are supplied through suitable pipe lines 25,26 and 21 with fresh water if so desired. The pipe line 23 may beextended beyond the nozzles 22 to provide a set of nozzles 28 with weakblack liquor which may be sprayed on the entrance lip of the vet to keepthe lip clean. The pipe.

line 23 may also be utilized to supply a set of nozzles 29 with weakblack liquor which maybe sprayed on the outer surface of the knotterscreen IE to dislodge particles from the openings therein.

As shown in the drawings, a vacuum sector 30 -(which may convenientlycomprise a plurality of separate sections not illustrated) is arranged30 within the drum 2!. It is necessary for success- This effect issufficient to render the entire mass susceptible I v This is necessaryto enable a complete drawing to foamingso long as the particles are notre-' moved. The efiect of forming localized channels will existregardless of the character of the oversized particles but if theparticles be porous,

pollution will result from the presence of the particles themselves.This effect will occur wherever the condition occurs, whether in pulpsor in other suspensions.

It has long been the practice to remove knots and chips from sulphitepulp prior to washing;

but this practice has been considered impossible are of a porous nature.

In the case of kraft board mills using diffusers to wash the pulp, ourprocess offers an excepful washin of this type to have the vacuum sectorunderlying the drum extend for several degrees on the trailing sidebeyond the point where the spray from the nozzles is applied to the mat.

through of the liquor supplied by means of the sprays. It wouldaccordingly be a difficult (though not necessarily impossible job) todesign a single drum which would accommodate a series of washingstogether with intermediate application of liquid as hereinafter morefully dis closed. The use of a single drum would, however,

' be difficult since not much more than fifty per tional advantage aswill be hereinafter described.

Y We have found that a knot remover may successfully be operated inadvance of the washer provided it be driven at a speed only sufficientto permit .clearance of the holes in the knot removing screen and thusto avoid clogging.

.Fig. 1 illustrates schematically our general arrangement of equipmentfor carrying out our operate such a screen. We have found that an javerage peripheral speed of 5 feet per minute is about the maximum speedat which a knotter can be operated in this position without foaming andthis maximum speed is independent of either the length'or diameter ofthe knotter screen.

cent of the surface of the drum is available for spraying and suckingaction, and to accommodate the number of washings and liquorapplications illustrated in the diagram in Fig. 1 would require a verylarge drum and would therefore involve considerable engineeringdifficulties. Thesuction applied through sector 30 draws the weak blackliquor and fresh water through the pulp, thereby mixing the weak blackliquor and water with the strong black liquor originally in the pulp.The mixed liquors pass through a. pipe line 3| to a foam-trap 32 andpass at once to the black liquor storage tank ll. As a matter ofconvenience, an additional line 33 may take a portion of these liquorsto a. separate foam trap ll so that the liquors may be fed to the weakliquor storage system for the purpose of adjustin the concentration ofchemicals in that system.

As the pulp approaches the trailing edge of the vacuum sector 30 in thefirst drum the suction is discontinued and the pulp is doctored off bydoctor 35A onto a conveyor 35 which consists of an endless belt run oversheaves 36 and 31. This belt is porous and a. water box 38 extendsacross its width and carries a spray line 39 supplying fresh hot water.in the amount of from to of a pound per poimd of stock (air-dry) passingover the conveyor. The water box or the jets may be used to apply areagent of some sort to the material being washed. For example, in thecase of pulp, the applicationofcaustic soda will be beneficial in asubsequent bleaching process.

vat 53 whence it is e stock storage tank.

This could, of course, be applied by means of any of the sprays playingon the washer drums. To.

supplied by means of sprays per unit weight of solids washed isenormously greater due to the necessity for drawing it through the massthan is the case in the-water box 38 which merely saturates the masspresented to the box. Our method and apparatus are therefpre decidedlymore.

economical. It will be noted that sprays 40 and 4| are appliedrespectively to the pulp mat as it leaves the drum 2| and as itapproaches the drum 42. These sprays'may have the same function as thewater box 38. In practice a set-up may be maintained by the use of -allthree or of any one. -It may in some cases be advantageous to dispensewith the water box 38 and to rely on one or both of the sprays 4!! and4| adjacent the respective drums. Such an arrangement will per mit theuse of a solid instead of a foraminous belt. A solid'belt ismechanically easier to guide and adjustthan a foraminous belt.

-The blanket of pulp passes from the conveyor '35 over a foraminousguide or support 353 to a second drum 42 where it is picked up by asuction applied to a segment 43. A series of nozzles 44 supplied by a'pipe line45 extends across thewidth Off the second drum 42 and suppliesfresh hot water at the rate of about 1 pound of water A for each pound(air dry) of pulp passing over the drum. Th s water is drawn through thevacuum I segment 43,'through a pipe line 46, to the aforementioned foamtrap 34 and the residual liquor .is forced by a pump 41 through a pipe48 to a weak liquor storage tank 49. A second vacuum segment 56 isprovided in the second drum 42. As the pulp passes over the secondsegment 50 it is subjected to a spray of fresh cold water supplied drum56.th'e pulp will pass under a fresh water spray 6| which will be drawnthrough the blanket of the pulpmat as previously described. The

usual operation would be for the liquor drawn throughthe second drum 55-to be used as a spray for washing the stock on the first drum 54 and forthe liquor drawn through the third drum 56 to be used as a spray forwashing the stock on the second drum 55.

Such arrangements for counter current washing are old and well-known,and it isconsidered unnecessary to illustrate all of the piping inconnection with the disclosure of Fig. 2.

Fig. 3 introduces still another variation. Here a drum 62 operates in avat 63 which would receive the stock from the knotter in the manner asthe drum 2| of Fig. 1. The stock picked up and passing over the drum 62will be sprayed and the liquor drawn through the stock and delivered tothe black liquor storage tank. The web 64 of stock leaving the drum 62is picked up on a conveyor belt 65 which passes over sheaves 66 andtion. boxes would be supplied by liquor drawn by nozzle 5| in theaggregate amount of about 1 pound per pound (air dry) of pulp. Theliquor drawn through the second segment has so low a concentration ofchemicals (less than 1 'Baum) that it is. not worth salvaging in theevaporation process, and this liquor is therefore discarded via a pipeline 52. The stock leaving the second drum goes-to a piped to the normalbrown A variation of the above arrangement of washer drums isillustrated in Fig. 2 in which three drums 54, and 56 are'provided. Inusing this system stock would be fed from the knotter to a vat 51underlying the flrstdrum 54.and corresponding through the second suctionbox and the spray above the second suction box would be supplied withliquor drawn through the third suction box. If desired, 'a suctionsegment 10 can be placed in the sheave61 and a fresh water spraydirected on it in which case the liquor drawn through the suctionsegment 10 would supply the spray over the third suction box. In theabsence of such asuction segment the spray over v the third suction boxwould probably be supup from slurry by any of the several drums.

to the drum 2| .illustrated in Fig. .1. In passing over the first drum54"the stock will be supplied with weak black liquor through sprays 58as in v the case of the drum 2| of Fig. 1. From the first drum 54 thestock in the form of a web 59 in unbroken condition will be transferredto a second drum ,5 which will, however. pick up the oppo-. 7 s te faceof the web. In passing over a portion of the surface of the second drum55 the stock' will pass a spray supplying either weak liquor or freshwater which will be drawn through the blanket of pulp just as weakliquor is drawn through the blanket of pulp by the second drum 43 ofFig. 1. The sprays operating on the drum 55 will usually be suppliedwith'liquor drawn through the pulp passing over the third drum 56 now tobe described.

The vacuum operating within the second drum 55 will be cut off at apredetermined point on the drum 42.

plied with fresh water.

The feature which all of these various forms have in common is'that thestock is not broken up and diluted intermediate the several washingstages with the result that all of the stored liquor at every stage maybe maintained considerably more concentrated than would be the case weresuflicient water added to the system to dilute the stock to therequisite point for being picked It will be understood that suitabledoctor blades and guides may be provided to remove the mat from and toguide the same into contact with the washer drums.

Referring back to Fig. 1, it will be obvious that the amount of, wateradded by means of the water box 38 is negligible in comparison with theamount of water which would have to be added to dilute the mat comingfrom dru'm 2| to a consistency whichwould enable the pulp suspension tobe picked up from slurry and'handled by the Accordingly, theconcentration of chemicals extracted by the drum 42 through the pipeline 46 is very much greater than would otherwise be the case and theconcentration of the entire weak liquor system is much greater. Sincethe weak liquor system supplies the wash liquor to the drum 2| theconcentration of chemicals in the black liquor storage tank is very much.or 99% water.- present 19 pounds of water foreach pound of" retarded bythe screen I" mousf amount of liquor which; wouldhave to be addedl'to"the mat 'coming' from one stage in order to fit itfor-pick-upoperations by the subsequent stage; as wellas by ellminatingthe quantities of air which would be eu rllbped in the course of suchdilution. For :example'fthe consistency of sized chips accepted by .thescreen I08 will have 5- lost the characteristics which render themharmful in the washingprocess. .These finely divided 1 the stock as itcomes fromone stage Isflrobably about 95% water, while'- thej; ccnsistency of the pulp to enable the drum to pickrit-upfandhandle it wouldhave to be m the neighborhood of 98% At the 95% consistency there arestock, but at the 99% :consi'stencytherefare 99 pounds of waterfor'eachpound 'otfstock. In

other-words, for every-pound ofipulpvthere must be added 80'pounds ofwater in1' irder'to dilute it for proper presentation "toeaich stage ofthe washer. The pulp's and piping? are therefore called upon to handlealmost four times as much water as is the case through the use'of ourinvention. The cost of the equipment and the amount of power and thetask of ultimate evaporation of the black liquor are all thereforeapproximately four times as high by present methods as they are when ourinvention is used. In

large scale operations involving hundreds of tons of pulp per day thisdifference is of enormous importance.

The arrangement shown in Fig. 4 is particularly useful in connectionwith kraft board mill operation, though its principles may be applied inother connections. Pulp and black liquor are blown from a digester Iinto a blow-pit IIII where it may be diluted as desired. A suggestedmeans ofdilution is to supply black liquor to the blow-pit from theblack liquor storage tank I I02.

' Stock is pumped from theblow pit to a knot screen I03 from which theaccepted stock goes through a pipe line I09 to a washing device I04.

v This device may be either a washer of the type illustrated in Fig. 1or a diffuser of the type very largely used in kraft pulp mills, theprecise type of washer being immaterial, though naturally thisarrangement offers an additional advantage in connection with a washerof the type shown in Fig. 1 for reasons hereinabove discussed. l

The knots, uncooked chips'and other oversized particles separated fromthe mass by the knot screen are fed to a refiner I0lwhich may be of anydesired type andwhich acts to crush and otherwise disintegrate the lumpsinto particles of a size compatible with the stock accepted by the knotscreen. From the refiner I the disintegrated lumps proceed to a screenI00 which accepts only'such finely sized particles as will be compatiblewith the accepted pulp. These particles pass through a pipe line I01 andJoin the stream of accepted stock'going through line I" to the washerordiiluser.v The oversized particles a line I back to the stream ofoversized par-' ticles going from the lmot screen I0! to the refiner I"where they are once more subjected to a disintegrating process. A

While this system would not be satisfactory for l the production of highgrade papers it oilers very considerable advantage for the production ofcoarse papers and board and by removing the 76 and uncooked chips fromsuspension of pulp 2 oversized particles it increases the efilciency ofthe washer since after washing there is no source ;of chemical pollutionof the stock. Aspreviously mentioned, the presenc of relatively coarseknots 5 and lumps would make it impossible to obtain a effective and thestock is ready for delivery to the refiners and then to the paper orboard machine.

It will be understood that the kno s and overfibrous particles may bewashed clean. in the usual pulp washing device employed. a I In certainmills and in the manufacture of certain grades of paper it may not'1bedesirable to return the accepted comminuted knots and uncooked chips tothe pulp'system. In such cases the accepted fines .from screen I06 maybe di- 'verted from the system'and utilized or otherwise disposed of inany suitable manner.

While our process has been described with particular reference to thewashing of alkaline pulps, it is obviously applicable to the washing ofany form of suspension such as sewage, bagasse and 0 other mixtures. Wedo not, therefore, intend in any way to limit the scope of our inventionex-' cept as set forth in the subjoined claims which are to be broadlyconstrued. l

We claim:

l'. The method of screening and washing pulp digested by an alkalineprocess and suspended in black liquor resulting from said digestingproc-.

e'ss which comprises passing said pulp suspension over a rotary knotterfor the separation of knots 40 and uncooked chips from thesuspension ofpulp particles of acceptable size, rotating said knotter at a peripheralspeed not exceeding five feet per minute whereby substantially ,to avoidfoaming of the black liquor in which saidpiilp lawspended, removing saidsuspension-fol f pulp par- 2. The method of screening and washing-pulpdigested by an alkaline process and suspended in black liquor resultingfrom said digesting process which comprises passing saidpulplsuspen'sion"over a rotary knotter for the separation of knots and uncooked chipsfromithe suspension of pulp particles of acceptable size, rotating saidknotter at a peripheral speed not exceeding flve feet per minute'wherebysubstantially to avoid foaming oi the black liquor in which said pulp issusoo pended, passifig said suspension or pulp par-' ticles ofacceptable size and free from knots and chips from said kn'otter to aseries of washing stations, washing saidpulp particles by subjectin: thesame to suction and spraying by wash to liquid in each of said washingstations, and using fresh hot water as the wash liquid in at least the mg1- last of said washing stations whereby to remove sTibstantially allof the black. liquor inwhich said pulp initially was-suspended;

7 3. The method of screening washing pulp digested by an'alkaline'process and suspendedin black liquor resulting from, saiddigesting process which comprises, passingsaid pulp v suspension over arotary knotter for the separation of knots at a peripheral speed notexceeding five feet per minute whereby substantially to avoid foaming ifthe black liquor in which said pulp is susp :nded, forming said pulpparticles of acceptable sizefree from knots and chips into' a mat,passing said mat through a succession of washi steps in which one sidethereof is subjected to suction while the other side thereof is sprayedwith washing liquid, and finally passing said mat through a washingstation in which one side thereof is subjected to suction while theother side thereof is sprayed with fresh hot water whereb substantiallyto remove all of the black liquor in which said pulp initially wassuspended.

4. The method of washing pulp suspended in a liquid to removesubstantially all of said suspension liquid which comprises forming amat of said pulp upon the surface of a rotary washing screen by suctionacting to draw a portion of the suspension liquid from said pulp throughsaid screen, rotating said screen while applying suction to one side ofsaid mat and simultaneously spraying the opposite sideof said mat withwashing liquid, removing said mat from said rotary washing screen andpassing said mat without disintegrating the same into suspension in abody of liquid to a second rotary washing screen, spraying the firstnamed side of said mat with washing liquid while passing the same fromsaid first rotary washing screen to said second rotary washing screen,rotating the said second rotary washing screen while applying suction toone side-of said mat and simultaneously spraying the opposite side ofsaid mat with washing liquid, and removing said mat from said secondrotary Washing screen.

5. The method of washing pulpsuspended in a liquid to removesubstantially all-of said suspension liquid which comprises forming amat of said pulp upon the surface of a rotary washing screen bysuctionacting to draw a portion of the suspension liquid from said pulp throughsaid screen, rotating said screen while applying suction to one side ofsaid mat and simultaneously spraying the opposite side of said mat withwashing liquid. removing said mat from said rotary washing screen andpassing said mat without disintegrating the same into suspension infabody of liquid to a second rotary washing screen. spraying the firstnamed side of said mat with washingliquid while passing the same fromsaid first rotary washing screen to said second rotary washingscreen,-rotating the said second rotary washing screen while applyingsuction toone side of said mat and simultaneously spraying the oppositeside of said mat with washing liquid, the final washing liquid at leastcomprising fresh hot water, and removing said mat from saidv secondrotary washing screen.

6. The method of screening and washing pulp digested by an'alkaiineprocess and suspended in black liquor resulting from said digestingprocess which comprises passing said pulp suspension over a rotaryknotter for the separation of knots and uncooked chips from thesuspension of pulp particles of acceptable size, rotating said knotterat a peripheral speed not exceeding five feet per minute wherebysubstantially to avoid foaming of the black liquor in which said pulp issuspended, passing said suspension of pulp particles of acceptable sizefree fromknots and-chips from said knotter into a vat, forming a matfrom said pulp suspension on'the surface of a rotary washing screen,subjecting' one side of said mat to suction as the same passes over saidwashing screen whereby to remove a portion of the suspension liquortherefrom, simultaneously spraying the opposite side of said mat withweak black liquor, removing said mat from said rotary washing screen andpassing said mat without disintegrating the same into suspension in abody of liquid to a second rotary washing screen, spraying thefirstnamed side of said mat with fresh hot water as the mat is beingtransferred from said first rotary washing screen to said second rotarywashing screen, subjecting said first-named side of said mat to suctionas the same passes over said second rotary washingscreen and sprayingthe opposite side of said mat first with a weak black liquor andthereafter with fresh hot water while suction is applied to saidfirst-named side,

said last-named weak "black liquor being obtained from said' l astspraying ste in which said mat is sprayed with fresh hot water, andpassing'said mat to a vat in which it is disintegrated into suspensionin a suitable liquid whereby to form a pulp suspension substantiallyfree from the alkaline cooking liquor in which said pulp was digested. I

. FRANK J. LANG.

GEORGE R. JAEGER.

